Free atmospheric CO₂ enrichment increased above ground biomass but did not affect symbiotic N₂-fixation and soil carbon dynamics in a mixed deciduous stand in Wales

Abstract. Through increases in net primary production (NPP), elevated CO₂ is hypothesized to increase the amount of plant litter entering the soil. The fate of this extra carbon on the forest floor or in mineral soil is currently not clear. Moreover, increased rates of NPP can be maintained only if forests can escape nitrogen limitation. In a Free atmospheric CO₂ Enrichment (FACE) experiment near Bangor, Wales, 4 ambient and 4 elevated [CO₂] plots were planted with patches of Betula pendula, Alnus glutinosa and Fagus sylvatica on a former arable field. After 4 years, biomass averaged for the 3 species was 5497 (se 270) g m⁻² in ambient and 6450 (se 130) g m⁻² in elevated [CO₂] plots, a significant increase of 17% (P = 0.018). During that time, only a shallow L forest floor litter layer had formed due to intensive bioturbation. Total soil C and N contents increased irrespective of treatment and species as a result of afforestation. We could not detect an additional C sink in the soil, nor were soil C stabilization processes affected by elevated [CO₂]. We observed a decrease of leaf N content in Betula and Alnus under elevated [CO₂], while the soil C/N ratio decreased regardless of CO₂ treatment. The ratio of N taken up from the soil and by N₂-fixation in Alnus was not affected by elevated [CO₂]. We infer that increased nitrogen use efficiency is the mechanism by which increased NPP is sustained under elevated [CO₂] at this site.